1. Field of the Invention
The disclosures herein generally relate to memory devices, and particularly relate to a nonvolatile memory device.
2. Description of the Related Art
Nonvolatile semiconductor memory devices include flash EEPROMs employing a floating gate structure, FeRAMs employing a ferroelectric film, MRAMs employing a ferromagnetic film, etc. EEPROMs typically require a transistor having a special structure comprised of a floating gate. FeRAMs and MRAMs, for example, achieve nonvolatile storage by use of a ferroelectric material and a ferromagnetic material, respectively, thereby requiring a film made of these respective materials. The need for such a transistor having a special structure and the need for such a film made of special material are one of the factors that result in an increase in the manufacturing costs.
A nonvolatile data storage function may alternatively be provided by utilizing an MIS (metal-insulating film-semiconductor) transistor as a memory cell transistor. That is, an MIS transistor having the same structure as ordinary MIS transistors that are typically used for conventional transistor functions (e.g., switching function) may be employed as a memory cell transistor. Such a memory cell transistor is advantageous because neither a special structure nor a special material is required.
PermSRAM, which utilizes an MIS transistor as a memory cell transistor, is commercially available. PermSRAM stores data in an MIS transistor by utilizing a hot-carrier effect. The hot-carrier effect refers to the injection of electrons into the insulating film (i.e., oxide film) and/or sidewalls of the transistor. The presence of electrons trapped in the insulating film and/or sidewalls causes a rise in the threshold voltage and a drop in the drain current. The amount of drain current of a given MIS transistor may be measured to determine whether this transistor has electrons trapped in the oxide film and/or sidewalls. The presence of trapped electrons may represent one-bit data “0”, for example, and the absence of trapped electrons may represent one-bit data “1”, for example.
The hot carrier effect is generally regarded as being substantially irreversible. Hot holes may be injected to the insulating film and/or sidewalls to remove some of the trapped electrons, thereby mitigating the effect of trapped electrons on the threshold voltage and the drain current. However, all the trapped electrons cannot be removed, and the remaining trapped electrons still causes a rise in the threshold voltage and a drop in the drain current.